Influence of PH on the hydrolytic and acidogenic phases during anaerobic digestion of waste activated solids in a batch system

Fermentations of complex waste streams are sensitive to multiple environmental conditions, with pH being one key parameter. A laboratory-scale study was performed to describe the impact of pH on acidogenic fermentation of municipal wastewater treatment biosolids produced from a New Zealand nutrient removal plant. Duplicate batch fermentations were conducted, at two pH levels- pH 6 and 8. The effects of pH on hydrolysis and volatile fatty acid formation and purity were examined across a 15 day time series. It was expected that the pH 8 reactors would show a greater level of hydrolysis, but results indicated that volatile suspended solids destruction was greater at lower pH. Further, acidogenic formation of VFA was greater at low pH, also not an expected result. At pH6, methanogenic fermentation was suspected (but not confirmed) to impact on VFA productivity after 9 days fermentation, a phenomenon not observed at pH8.

Under more alkaline conditions, the kinetics of reaction did appear slower, with indications that the full extent of fermentation had not been reached by the end of the 15 day study. Acclimation of the microbiological community to the increased pH conditions may thus have been a significant factor in the hydrolysis/acidogenic efficacy of the system.

Certainly the results confirm that operational pH does have a significant impact on the acidogenic fermentation of biosolids, even across a moderately narrow span of pH, providing considerable potential for manipulation of product formation from these complex organic solids.